1. Field of the Invention
The present invention relates in general to a roller turret, a method of manufacturing the roller turret, a roller turret cam index device, and a roller turret type rotary motion transmitting device, and more particularly to techniques for improvement of accuracy of pitch of rollers used in the roller turret.
2. Discussion of the Related Art
Generally, a roller turret includes a turret body rotatable about an axis thereof, a plurality of roller shafts provided on the turret body, and a plurality of rollers rotatably supported by the respective roller shafts. Each of the roller shaft includes a stud portion fitted in a mounting hole formed in the turret body, and a roller support portion which is concentric or coaxial with the stud portion and which supports the roller rotatably. The roller support portion of the roller shaft is disposed outside the turret body, so that the roller also disposed outside the roller is rotatably supported by the roller support portion.
Such a roller turret may be combined with a globoidal cam, a barrel cam, or a plate cam unit which is a combination of a plurality of plate cams (usually, two plate cams). The roller turret cooperates with the globoidal cam, the barrel cam and the plate cam unit, to constitute a globoidal cam index device, a barrel cam index device, and a parallel cam index device, respectively. The roller turret combined with the globoidal cam is generally called "a roller gear". In the present application, the roller turret combined with the globoidal cam is referred to as "a spur roller gear" as distinguished from "a face roller gear", which is the roller turret combined with the barrel cam. Those roller turrets are referred to as the spur roller gear and the face roller gear, in view of their similarity in configuration to a spur gear and a face gear. The roller turret combined with the plate cam unit, on the other hand, is referred to as "a zigzag roller turret", in view of a pattern of arrangement of the rollers. The globoidal cam index device, the barrel cam index device and the parallel cam index device are collectively referred to as "a roller turret index device".
In the spur roller gear, the roller shafts are disposed on the turret body so as to extend in the radial direction of the turret body such that the axes of the roller shafts are perpendicular to the axis of rotation of the turret body. In the face roller gear and the zigzag roller gear, the roller shafts are disposed on the turret body so as to extend in the axial direction of the turret body such that the axes of the roller shafts are parallel to the axis of rotation of the turret body. The globoidal cam, the barrel cam and the plate cam units are collectively referred to as "a roller turret cam". Each of these roller turret cams includes a dwelling portion and an indexing portion with which the roller turret engages during rotation of the roller turret cam, so that the roller turret is held stationary while it is held in engagement with the dwelling portion, and is rotated while it is held in engagement with the indexing portion.
The spur roller gear is combined with a double enveloping worm to provide a spur roller gear type speed reducing device, while the face roller gear is combined with a cylindrical worm to provide a face roller gear type speed reducing device. Such a double enveloping or cylindrical worm has a thread which corresponds to a rib formed on the globoidal and barrel cams and which does not include a dwelling portion and therefore has a constant lead. A rotary motion of the worm at a constant speed will cause a rotary motion of the spur or face roller gear at a comparatively lower constant speed. The speed ratio of the spur or face roller gear with respect to the double enveloping or cylindrical worm can be increased to 1.0 by increasing the number of starts of the thread or tooth of the worm. In this case, the spur or face roller gear and the worm cooperate to constitute a spur or face roller gear type rotary motion transmitting device. Further, a spur or face roller gear type speed increasing device may be obtained where the spur or face roller gear is used as an input-side device while the double enveloping or cylindrical worm is used as an output-side device. The speed reducing device, rotary motion transmitting device (in a narrow sense) and speed increasing device, which have been described above, are collectively referred to as a rotary motion transmitting device in a broad sense. The spur roller gear type rotary motion transmitting device and the face roller gear type rotary motion transmitting device are referred to as a roller turret type rotary motion transmitting device. Where the roller gear type rotary motion transmitting device is used in combination with a drive device capable of rotating the worm (or roller gear) such that the worm is stopped at a desired angular position, the roller gear (or worm) can be stopped at a corresponding angular position.
In the conventional roller turret cam index device, it is difficult to improve the indexing accuracy of the roller turret, namely, the accuracy of stopping the roller turret at the desired angular position. Accordingly, the conventional roller turret type rotary motion transmitting device suffers from a problem of difficulty to improve the accuracy of rotary motion transmission (the accuracy of relative angular position of the roller turret and the worm). This difficulty is primarily due to difficulty to improve the accuracy of pitch of the rollers in the roller turret.
The mounting holes in which the roller shafts are partly received are formed in the turret body such that the roller shafts are equiangularly arranged and spaced apart from each other in the circumferential direction of the turret body. Even if the mounting holes are formed by machining with a high degree of accuracy of their positions, the mounting holes inevitably have an angular positioning error of about .+-.5 seconds. Usually, the stud portion of each roller shaft is press-fitted in the mounting hole with some amount of interference. Since the amount of interference is on the order of single-digit microns (p), the roller shafts whose diameters of the stud portions have different amounts of deviation from the nominal value must be fitted in the appropriately selected mounting holes whose diameters also have different amounts of deviation from the nominal value. That is, the diameters of the stud portions of the roller shafts which have been produced are classified into a plurality of diameter groups. On the other hand, the diameters of the mounting holes formed in the roller turret are measured. The combinations of the roller shafts and the mounting holes are determined so that the amount of interference between the stud portion of each roller shaft and the corresponding mounting hole is held within an optimum range. However, the matching of the diameters of the roller shafts with respect to the diameters of the mounting holes is a cumbersome procedure. Although the stud portion of the roller shaft may be fitted in the mounting hole with a clearance within a predetermined tolerance, this clearance fit is likely to deteriorate the accuracy of pitch of the rollers and reduce the strength of fixing of the roller shaft in the mounting hole. Alternatively, the stud portion of the roller shaft may be press-fitted in the mounting hole with a sufficiently large amount of interference, an excessively large amount of interference therebetween may lead to inclination or misalignment of the axis of the roller shaft relative to the centerline of the mounting hole, resulting in lowering of the accuracy of pitch of the rollers.
Further, it is impossible to establish a perfect alignment of the axis of the turret body about which the mounting holes are equiangularly spaced apart from each other by a machining operation on the turret body, with the axis of rotation of the end product, namely, the axis of rotation of the index device or rotary motion transmitting device. That is, the axis or center about which the mounting holes are equiangularly spaced apart from each other may more or less deviate from the axis of rotation of the turret body, and this deviation results in an angular positioning error of the mounting holes, and consequently causes deterioration of the roller pitch accuracy of the end product. If the turret body consists of a plurality of pieces which are fixed together, the misalignment or deviation of the two axes indicated above may be reduced to within about 2-3 microns. However, this solution is not satisfactory.
Further, the dimensional errors of the components associated with the rollers may also cause the roller pitch error of the roller turret. Where the roller is mounted on the roller support portion of the roller shaft via a needle bearing interposed therebetween such that the roller support portion and the roller respectively function as an inner race and an outer race of a roller bearing, for instance, a variation of the outside diameter of the roller support portion and variations of the inside and outside diameters of the roller will cause an error of the roller pitch of the roller turret. Therefore, it is required to minimize the overall diameter error of an assembly of the roller support portion, needle bearing and roller. However, the overall diameter error of that assembly cannot be actually reduced to a value smaller than 2-3 microns.
In addition, misalignment of the actual axis of rotation of the turret body with respect to the nominal axis due to inaccuracy of bearings used for rotatably supporting the turret body will cause the same result as if the roller turret had a roller pitch error. To avoid this problem, it is required to use high-precision bearings for supporting the turret body with high accuracy. In this case, the cost of manufacture of the roller turret assembly is undesirably increased. In some cases, the amount of misalignment cannot be sufficiently reduced even if expensive high-precision bearings are used for rotatably supporting the roller turret.
As described above, there are various causes for the roller pitch error of the roller turret. Each of the errors arising from the respective causes must be minimized. For instance, it is required to improve the accuracy of machining to form the mounting holes, the accuracy of mounting of the roller shafts on the turret body, and the dimensional accuracy of the components associated with the rollers. In this instance, however, the roller pitch error is still relatively large due to a cumulative error including the errors generated by the different causes. For instance, the roller turret of the roller turret cam index device inevitably suffers from an indexing error or angular positioning error of about .+-.10 seconds. Since the cost of manufacture of the roller turret progressively or exponentially increases with an increase in the required degree of accuracy, the roller pitch error of the roller turret cannot be further reduced at a practically acceptable cost.
The globoidal cam index device, barrel cam index device, spur roller gear type rotary motion transmitting device and face roller gear type rotary motion transmitting device have a further problem that the angular positioning or indexing accuracy is deteriorated due to vibration of the spur or face roller gear, which is generated due to a clearance or gap between the outer circumferential surface of the rollers and the rib of the globoidal or barrel cam or the thread of the worm. Each roller of the roller gear is moved in a groove defined by two portions of the rib of the globoidal or barrel cam or the thread of the worm, which two portions are spaced apart from each other in the axial direction of the cam or worm. To permit the roller to move in the groove in rolling contact with the mutually facing surfaces of the walls of the groove, it is necessary to provide a clearance between the surfaces of the walls and the outer circumferential surface of the roller. This clearance causes vibration or operating noise of the roller gear, which deteriorates the angular positioning or indexing accuracy and the rotary motion transmitting accuracy of the index device or rotary motion transmitting device.